Windows Phone - ListBox [UnSelectAll]

Podemos facilmente selecionar programaticamente todos os itens de um ListBox [múltipla escolha] através do método [SelectAll] porém, para a plataforma WP, não existe o método [UnSelectAll]. Sendo assim, vamos criar um!...
Vamos desde o início criar nosso projeto no Visual Studio para isto, conforme ilustrado abaixo:





Com o projeto inicial criado, substitua o conteúdo do [Code-Behind - MainPage.xaml.cs] pelo que segue:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Navigation;
using Microsoft.Phone.Controls;
using Microsoft.Phone.Shell;
using PhoneApp.Resources;

namespace PhoneApp
{
    public partial class MainPage : PhoneApplicationPage
    {
        // Constructor
        public MainPage()
        {
            InitializeComponent();
            this.MultiResultList.Items.Add("Item 1");
            this.MultiResultList.Items.Add("Item 2");
            this.MultiResultList.Items.Add("Item 3");
            this.MultiResultList.Items.Add("Item 4");
            this.MultiResultList.Items.Add("Item 5");
        }

        private void chkSelectAll_Click(object sender, RoutedEventArgs e)
        {
            var chk = (CheckBox)sender;

            if (chk.IsChecked.Value)
                this.MultiResultList.SelectAll();
            else
            {
                for (int i = this.MultiResultList.SelectedItems.Count - 1; i >= 0; i--)
                {
                    this.MultiResultList.SelectedItems[i] = null;
                }
            }
        }
    }
}

E também substitua seu XAML (MainPage.xaml) com o que segue:

<phone:PhoneApplicationPage
    x:Class="PhoneApp.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
    xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeNormal}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    SupportedOrientations="Portrait" Orientation="Portrait"
    shell:SystemTray.IsVisible="True">

    <!--LayoutRoot is the root grid where all page content is placed-->
    <Grid x:Name="LayoutRoot" Background="Transparent">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <!--TitlePanel contains the name of the application and page title-->
        <StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="12,17,0,28">
            <TextBlock Text="MY APPLICATION" Style="{StaticResource PhoneTextNormalStyle}" Margin="12,0"/>
            <TextBlock Text="page name" Margin="9,-7,0,0" Style="{StaticResource PhoneTextTitle1Style}"/>
        </StackPanel>

        <!--ContentPanel - place additional content here-->
        <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">

            <ScrollViewer  BorderThickness="1">
                <StackPanel Margin="0,0,0,17" Width="432">

                    <Grid>
                        <Grid.RowDefinitions>
                            <RowDefinition />
                            <RowDefinition />
                        </Grid.RowDefinitions>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="432" />
                        </Grid.ColumnDefinitions>

                        <CheckBox Name="chkSelectAll" Grid.Row="0" Foreground="{StaticResource PhoneForegroundBrush}" Click="chkSelectAll_Click" >
                            <TextBlock Text="Selecionar Todos" TextWrapping="Wrap">
                            </TextBlock>
                        </CheckBox>

                        <ListBox Grid.Row="1"
                            Name="MultiResultList" 
                            SelectionMode="Multiple" 
                            Height="510" 
                            Margin="0,0,0,-510"
                            FontSize="{StaticResource PhoneFontSizeExtraLarge}">
                        </ListBox>

                    </Grid>

                </StackPanel>
            </ScrollViewer>
            

        </Grid>

    </Grid>

</phone:PhoneApplicationPage>

E verá que, ao clicar no CheckBox, todos os itens da lista serão marcados / desmarcados. O ponto importante a notar no code-behind é o loop reverso feito para desmarcar cada opção selecionada do ListBox

for (int i = this.MultiResultList.SelectedItems.Count - 1; i >= 0; i--)
{
    this.MultiResultList.SelectedItems[i] = null;
}

   

[]s!